The Ghana international arrived at Stamford Bridge from Olympique Lyonnais in 2005 and has since made 116 Premier League appearances, but has not played for his club this year because of a torn hamstring.
He added: "I am looking forward to coming back and enjoying my football and winning more trophies with the club. That is why I am here. I think it is one of the most important contracts in my career." Chelsea won both the Premier League and FA Cup this season and will start next term in the UEFA Champions League group stage.